1. Field of the Invention
The present invention relates to a white balance correction circuit, and more particularly, it relates to a white balance correction circuit which is applied to a color video camera or the like.
2. Description of the Prior Art
In general, a fixed system and an automatic-tracing system are employed for white balance correction circuits of color video cameras. The fixed system white balance correction circuit is simpler in structure than the automatic-tracing system one, and is generally applied to various types of color video cameras. However, the fixed system white balance correction circuit inevitably requires complicated white balance set operation, while the same cannot follow color temperature change of light on the object since a white balance correction state is fixed.
Therefore, white balance correction circuits of the automatic-tracing system are increasingly employed in recent color video cameras. Such white balance correction circuits of the automatic-tracing system are mainly provided in two systems. In the first system, a plurality of photodetector devices provided with color filters for different colors are arranged to face an object to detect the color temperature thereof by comparing output levels of the photodetector devices thereby to perform white balance correction in an open loop on the basis of the detected color temperature, as disclosed in Japanese Patent Laying-Open Gazette No. 90886/1983, for example (the circuit of such a system is hereinafter referred to as first automatic-tracing system white balance correction circuit).
In the second system, the gains of red and blue signals are controlled in a closed loop so that average levels of both of color difference signals (R-Y) and (B-Y) included in color video signals are zeroized, as disclosed in "National Technical Report" Vol. 31, No. 1, Feb. 1985, pp. 98-102, for example (the circuit of such a system is hereinafter referred to as second automatic-tracing system white balance correction circuit).
However, although the aforementioned first automatic-tracing system white balance correction circuit operates relatively stably operates with respect to color temperature change of light on the object, accuracy of white balance correction is not sufficiently high since this circuit is in open loop control and the photodetector devices are different in spectrum characteristics from an image pickup element. In other words, the first automatic-tracing system white balance correction circuit may cause slight errors, although the same hardly causes significant errors. Further, the first automatic-tracing system white balance correction circuit has low photosensitivity and cannot correctly operate under low illuminance. Therefore, white balance correction is generally inhibited under low illuminance.
On the other hand, the second automatic-tracing system white balance correction circuit has high correction accuracy due to the closed loop control, whereas the same operates to fade colors on the object when imbalance of color is caused on the object. This is because the second automatic-tracing system white balance correction circuit is adapted to control the gains of the red and blue signals so that average levels of the color difference signals are always zeroized. Thus, although the second automatic-tracing system white balance correction circuit precisely executes white balance correction under prescribed conditions, the same may significantly malfunction depending on conditions.